Developing a L2 stack is challenging. It is about balancing the certificabilityand the flexibility of the solution.
Certificability means that the stack shall not be altered once it has been TA approved. L2 behaviors shall be black-boxed so the integrators have not to deal with that level of complexitiy. Developing at the L3 shall preserve L2 from regression.
Flexibilitymeans that stack shall be adaptable to any merchant/acquirer contextcontexts. The regional nature of the payment is not well addressed by EMVCo specifications:
a) There is not enough guidance to abstract the EMV application selection process considering the need of particular selection mechanisms in some countries that are not tested during L2 test plan but addressed during end-to-end integrationscertifications
b) There is not no clear logiccal definition of the card processing. There For example, there is no interoperability defined between entry points point implementations and contactlless kernels, payment application and entry points, or even payment applications and contact kernels. Merchant projects integration make so any entry points could activate any kernels developed by a third part.
c) Merchant projects implementations reveal difficulties to implement specific requirements such as acquirer risk management
cd) Proprietary tags management is adressed from a card’s perspective but there is not specifications on how to manage them from a specific L2 to L3 serviceL3 perspective
Consequently, a L2 stack shall provide a flexible mean to extend certified behavior.
Agnos callbacks is an answer to address that problematic. To try to limit the complexity and the risk of adding card processing behaviors during a L3 development, Agnos proposes different callbacks (8) up to 8 callbacks that extend Agnos behavior without having to dig into L2 specifications:
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